The present invention relates to an image density control method for an electrophotographic apparatus and, more particularly, to an image density control method which forms at least two test patterns on the surface of a photoconductive element which differ a great deal in latent image potential from each other, senses values relating to image densities of the patterns or of copy images associated with the patterns, and, based on the sensed values, controls some parameters relating to image processing which have influence on image density.
In an apparatus for electrophotography or electrostatic recording, a latent image electrostatically formed on a photoconductive element by a given method is generally developed by a developer which is supplied from a developing unit and contains microscopic colored particles called "toner". Usually, a toner is charged to a polarity opposite to that of the latent image and electrostatically deposited on the latent image to turn it into a toner image. Because the toner itself is consumed by the development, it is necessary to maintain the toner concentration in the developer constant or to make the toner supply to the developing unit constant in quantity. This demand may be implemented by measuring a toner concentration in the developer, a density of the image developed by the toner, or their associated values, and, when the density is short, supplying the toner to the developing unit or controlling various parameters which effect the density.
The control of the type described above is disclosed in, for example, Japanese Patent Publication Nos. 43-16199/1968 and 47-18600/1972, Japanese Patent Laid Open Publication Nos. 53-12336/1978, 53-90940/1978, 53-92138/1978, 53-95042/1978, 53-95043/1978, 53-95044/1978, 53-106129/1978, 54-97038/1979 and 54-97044/1979, and U.S. Pat. No. 2,956,487.
In regard to such a control, Japanese Patent Application 56-80309 teaches a toner density control method which senses densities of developed images representative of white and black test patterns respectively, and supplying a toner to a developing unit in such a manner as to maintain the ratio between the sensed values constant. Further, Japanese Patent Application No. 56-184289/1981 discloses an image density control method including a pattern read control which facilitates the control over the timings for detecting white and black patterns.
In the above-stated Japanese Patent Application No. 56-184289/1981, for example, after an area of a photoconductive element exposed to a white pattern is developed, a light emitting diode (LED) of a density sensor is so controlled as to emit a quantity of light which gives a predetermined signal level indicative of a density of the white pattern, e.g. 4.0 V. Meanwhile, after an area of the photoconductor exposed to a black pattern is developed, the toner is supplied to a developing unit only when a signal level indicative of a density of the black pattern increases beyond a reference voltage, e.g. 1.6 V. Naturally, such reference voltages are not restrictive and may be replaced by others. In detail, if settings for energizing a density sensor is inadequate or settings or adjustment of an analog circuit for processing output signals of the sensor is inadequate, the sensor output level tends to shift to a higher side or to a lower side. For example, where the LED of the sensor is conditioned to emit a quantity of light somewhat higher than a reference value, an excessive amount of toner will be supplied resulting in scattering of the toner or like occurrence. Conversely, where the quantity of light emanating from the LED is somewhat smaller than the reference value, a carrier will be scattered due to short toner supply. Such occurrences are observed not only when the circuit setting or adjustment is improper but also when the patterns themselves are incomplete or the sensor is contaminated, for example.
Assume that the LED is connected to a 5 V voltage source, the resistance of a resistor connected in series to the LED is predetermined such that the quantity of light output from the LED is adjustable to make the reflection voltage from the background (corresponding to a white pattern sensed level) 4.0 V, and a toner is to be supplied when the reflection potential of the pattern portion is not lower than 1.6 V (corresponding to a black pattern sensed level). When the resistance of the resistor connected with the LED is lowered to increase the light emission by 20%, the reflection voltage from the background (white pattern) becomes 4.8 V and the toner supply threshold voltage for the pattern area (black pattern) becomes 1.92 V. Nevertheless, the density control will occur in the proper manner inasmuch as there still holds the proportional relation 1.6/4=1.92/4.8=0.4.
However, when the resistance of the resistor connected to the LED is further lowered to increase the light emission by 40%, the reflection voltage in the background does not reach 5.6 V but becomes (source voltage--saturation voltage of phototransistor). That is, assuming that the saturation voltage is 0.2 V, the reflection voltage in the background does not increase beyond 4.8 V and, instead, only the reflection voltage in the pattern area increases by 40%. Therefore, while the toner supply threshold voltage necessary for proper density control is 5.6.times.0.4=2.24 V, the actual voltage becomes 4.8.times.0.4=1.92 V and the toner supply begins in response to such a voltage. As a result, the toner density is controlled to a value higher than the predetermined desired density, which would not only increase the image density but also bring about problems such as fogging and scattering of the toner.
The controlling problems have been described taking for example a prior art method which focuses white and black patterns to a photoconductive element, senses densities of their toner images after development by means of a photosensor, and controls the toner supply based on a ratio between the sensed densities. However, other modes of image density control suffer from the same problems such as erroneous settings or adjustments of the sensor operation level, shifts of the sensing level due to contamination of the sensor, and defects in the patterns themselves, that is, unusual operations in the control of various parameters associated with a density control in unexpected situations.